To further characterize the molecular nature of odontogenic epithelial-mesenchymal interactions in rat embryo dental primordia, we plan to identify and resolve the nuclear composition and spatial organization of the tissue-specific genetic program for tooth formation in-vitro. Developing incisors dissected from 16-day old Wistar/Furth rat embryos will be maintained in organ culture with and without prior exposure to the thymidine analog 5-bromodeoxyuridine (BrdU), a drug which differentially, but non-toxically, arrests the program for odontogenic differentiation in an irreversible fashion. In order to resolve the dynamic molecular organization of the respective epithelium and mesenchyme nuclei, we first intend to utilize specific restriction endonucleases along with agarose slab gel electrophoresis to analyze the extent and distribution of methylated bases in nuclear DNA of control and BrdU-treated primordia. Other restriction enzymes and DNA affinity matrices of defined base-pair specificities will be used to accurately localize and identify the spatial distribution of BrdU in rat embryo DNA. Total epithelial and mesenchymal proteins, as well as the corresponding nuclear proteins will be characterized by 2-dimensional protein gel electrophoresis. Likewise, the effect of BrdU on the composition and arrangement of nuclear proteins in transcriptionally active and template-inactive chromatin will be analyzed by 2-dimensional electrophoresis. These findings will be compared to the nuclear presence and localization of a latent, endogenous retrovirus in the rat genome by nucleic acid hybridization to immobilized restriction enzyme-digested DNA.